The present invention relates to an air-fuel ratio control system for an internal combustion engine emission control system with a three-way catalytic converter, and more particularly to a system for an engine mounted on a car effective during a cold engine condition so as to improve the emission control effect and driveability of the car.
In a cold engine condition, evaporation of the fuel supplied by the carburetor of the engine decreases. Particularly, this is extreme during acceleration of the engine, since the engine induction vacuum is low in the wide-open throttle operating condition for acceleration. As a result, the fuel tends to adhere to the wall of the induction passage of the engine. Consequently, the air-fuel ratio of the air-fuel mixture to be induced in the engine increases (lean air-fuel mixture), which will cause stumbling of the engine operation and decrease the driveability of the car. In addition, if the throttle valve is closed immediately after such an acceleration, the fuel which has adhered to the wall of the induction passage is greatly evaporated by the high vacuum pressure in the induction passage. Thus, the mixture is excessively enriched by the evaporated fuel, which results in an increase of the amount of harmful constituents in the exhaust gases.
In a conventional air-fuel ratio control system, when the temperature of the cooling water of the engine is lower than a predetermined value, the air-fuel ratio of the mixture is controlled to a ratio smaller than the stoichiometric air-fuel ratio, that is a rich mixture. However, it is difficult for the control system to improve the driveability of the car without increasing the amount of unburned constituents of the exhaust gases.